Vacuum tube meter



Patented Jan. 20, 1948 VACUUM TUBE METER John M. van Beuren, Morrstown, and Elton Conrad, Boonton, N. J., assimors to Measurements Corporation, Boonton, N. J a corporation of New Jersey Orig nal application July 8, 1944, Serial No.

544,122. Divided and this application December 19, 1945, Serial No. 636,034

This invention relates to an electrical indicating device adapted for use in connection with measuring electrical currents of relatively low value, especially alternating currents.

The present application is a division of United States patent application, Serial #544,122, filed July 8, 1944', by John M. van Beuren and Elton Conrad.

One purpose of the invention is to provide an indicating instrument adapted to be substituted in lieu of a dynamometer type of indicator, for example in connection with such measurements as are made with Wheatstone bridges or direction-finding apparatus,

Instruments of the dynamometer type are subject to the difllculties of being comparatively fragile, or of lacking sensitivity if they are made to be more rugged. Another difliculty with such types of instruments is that when their characteristic response curve is made to have a sufl'iciently great slope at the point of null indication, the instrument likewise is very responsive to overloading, and easi'y damaged by such overload.

Another purpose of this invention is to provide an indicating instrument which will present a characteristic curve having a steep slope at the null point, while at the same time this characteristic curve flattens off when the null point has been passed by a moderate degree, thus avoiding the danger of overloading the actual meter employed therewith.

Another purpose of this invention is to provide an indicating instrument of the type described which can be employed at relatively high frequencies, extending into the region of megacycles.

Reference is now made to the accompanying drawing where Fig. 1 represents one embodiment of thisinvention where the current to be measured feeds the grids of electronic tubes at the indicating instrument.

Fig. 2 represents a characteristic response graph of this indicating device, compared with a similar graph of a dynamometer type device.

Referring now especially to Fig. 1, this device is here shown as used in conjunction with a Wheatstone bridge indicated at H], having two arms, I I and I2, preferab'y of equal impedance, an unknown impedance at l3, and a variable balancing impedance at Hi. The energy feeding the bridge is derived from a winding I 5, here indicated as the secondary windingpf a transformer Hi. -The output of the bridge is grounded at one lead, aslis conventional in the art and is fed to the input terminals l1 and I8 of an elec- 2 Claims. (Cl. 171-95) tronic amplifier I 9. This amplifier is preferably the same phase as the input.

so constructed that the output thereof will have The output terminals and 2| of this amplifier are connected via coupling condenser 22 to a discharge resistor 23. One terminal of this resistor is grounded at 24, and the other terminal 25 -is connected to the grids 28 of two electronic tubes, 21 and 28, so that these grids are connected in parallel with one another.

Eectronic tubes 21 and 28 are here shown as of the triode type, but other types of tubes may be substituted therefor, by making such suitable alterations as will be familiar to one skilled in the art. The cathodes 29 and 30 may be heated from any suitable source (not shown) and are connected to the terminals of a suitable balancing resistance 3|, and also to the terminals of a secondary winding 32, here shown as a portion of transformer IS. The energy serving to feed transformer I 6 may be derived from any suitable source and fed to a suitabe primary winding, here indicated at 16'. Resistor 3| is provided with a variable mid-tap 33, connected to a ground 34, the purpose of which will hereinafter be explained.

Tubes 21 and 28 contain anodes 35 and 36 respectively. Shunted between these anodes is the meter proper 31 of the indicator, which may be a sensitive direct current meter, such as a milliammeter. It is preferable that this meter be provided with a center null point so that indications may be read with current flowing therethrough in either direction. Anodes 35 and 36 are connected respectively through resistors 38 and 39 to a ground point 40. These resistors are so chosen as to have substantially identical values of resistance.

The device in Fig. l operates as follows. The windings l5 and 32 are respectively supplied with alternating currents which should, in the case of both windings, be of frequencies which are either identical or harmonically related to one another. Likewise, for the best results, these two currents should be in phase with one another. In order to obtain the relationship of currents just described, windings l5 and 32 may conveniently be placed as shown, upon a common transformer, but this showing is purely illustrative and the two windings may be fed from unrelated sources, provided that the respective currents derived from such sources bear to one another the relationship above described.

Wheatstone bridge [0 has been shown as one possible form of measuring device which may be,

used in connection with this invention. However, it is to be understood that other measuring devices may be employed in the general position occupied by the Wheatstone bridge here shown, the essential requirement being that it can be arranged that the output of such measuring devices shall be identical in phase and frequency with the input. Amplifier l9 may be of an suitable type. For example, a two-stage amplifier having a gain of about 1000 has been found suitable for use with a Wheatstone bridge, when the input of the bridge is of an order of magnitude of about 0.2 volt.

When tubes of types such as 6J5, 6C5, etc., are employed, a potential of about 6 volts has been found suitable for winding 32. When adjustable midtap 33 effectively grounds the center point of this winding, there will exist a potential of approximately 3 volts between the cathodes of the two tubes and the ground. In lieu of resistor 3| and midtap 33, transformer winding 32 may have a suitable midtap provided.

The anodes of the two tubes, being grounded through resistors 38 and 39, will then'be at a maximum instantaneous potential of approximately 3 volts with respect to the cathodes of the tubes. However, the flow of current through resistors 38 and will reduce such potential to an even lower value. In adjusting this indicator, it is preferred to supply a switch 4! which will directly ground the grids 25 at the point 24. With these grids grounded, midtap 33 is adjusted until meter 3! gives a null indication. This adjustment is provided in order to balance out the inequalities between the two tubes and the anode resistors, which inequalities inevitably will be found to exist,

in practice. For the best results, it is preferable that tubes 21 and 28 be chosen to exhibit substantially identical electrical characteristics.

When switch 4| is open, any alternating current potential fed by bridge 10 to amplifier IE will appear between grids 26 and ground 24. Since these grids are in parallel, such grid input must of necessity be in phase with the anode po tential in one tube, and therefore must be out of phase with the anode potential of the other tube. This will give rise to an unbalanced condition between the two anodes and thus will cause meter 31 to indicate in one direction or the other, ac-

cording to the relative direction of phase shift. If bridge Ill be balanced to give the desired measurement by null balance, current will no longer be fed to grids 26 and meter 3'! will again show a null indication, thus indicating that bridge III has been balanced. One important advantage of this device is that meter 31 not only shows departure from the desired balanced condition of the measuring device itself, but will indicate by the direction in which it moves from the null point, the sense or direction in which the measuring device departs from balance.

With indicating devices according to this invention, it is necessary that any changes in the current to be measured shall occur at a rate which will not be more than a fraction of the frequency of the alternating current used to energize the indicating device. Since there is practically no limit to the frequency which may be used for such energization, this limitation will rarely be found to be a serious one.

Due to the possibility of employing high frequencies with this indicating device, the field of possible applications for the device is greatly extended. For example, it may be used inmany cases where it has hitherto been necessary to employ a cathode ray oscillograph. One example of such use is the balancing of circuits carrying radio frequency currents, such as circuits employed with multiple antennae and the like.

Still another advantage of this measuring device is that it may be employed for the indication or measurement of high frequency currents de rived from sources where undesired strays or interfering signals are superimposed upon such currents. For example such currents may be those." collected from a receiving antenna, where static may be present. When used for such measurement, this device tends to discriminate against the strays and static signals, since such signals occur with random phase and frequency relation ships, thereb producing random movements of the indicating meter proper. As previously described, it is necessary that the current to be measured arrive at the two tubes with phase and frequency related to the phase and frequency of the local energy supply. Since strays or static signals would only by chance and only momentarily bear any definite relationships of the type just described, any effect produced by such strays upon meter 31 would, at the most, produce such random movements of the indicating element of the meter as could readily be distinguished from the comparatively steady indication of the meter caused by the currents the measurement of which is desired.

In Fig. 2, a conventional representation, with Cartesian coordinates, is shown, of the response close to the null point of this indicating device. Such response is indicated by curve 80. It will be seen that, close to the null point, the slope of this curve is extremely steep, thus indicating the sensitivity of this device used as a null indicator. At points 8i and 82, it will be seen that the curve flattens oil and becomes substantially parallel to the horizontal axis, thus illustrating the action of this indicating device when sub- Jected to overload. The flattening of the curve is due to the action of the two electronic tubes. to which the meter is connected. These tubes afford a "cushioning effect, since the maximum value of current which may reach the anodes of the tubes is limited by resistors 38 and 39. These resistors are here shown as having a value of 15,000 ohms each, but such value is, of course, purely illustrative. In any case, the current flow through these resistors will of necessity be kept within a certain maximum value, with the employment of a predetermined voltage to supply the anodes of the tubes.

Still referring to Fig. 2, curve 83 is shown by way of comparison. This latter curve represents the response around the null point, as given by the usual type of dynamometer. It will be seen that this curve is substantially a straight line. not only close to the null point, but for an indefinite distance in both directions therefrom. The dynamometer exhibits a response according to the well-known formula IE cos 0, while only the cenin lieu of using two separate electronic tubes, the various elements may be assembled within a single envelope, constituting a polyfunctional tube. While there has been described the use of this indicating device in connection with phase measurements, it will be apparent to one skilled in the art that measurement of amplitude can like- Wise be accomplished thereby. Therefore, the scope of this invention is limited only by the hereunto appended claims.

We claim:

1. Ind'cating device of the character described. including a pair of electronic amplifier tubes each including a cathode, an anode and a control grid, a source of alternating current connected between said cathodes, means for grounding the virtual electrical midpoint offsaid source, a d'rect current indicator connected between the anodes of 'said tubes, two resistors of substantially equal resistance connected one from each anode directly to the ground, whereby between said cathodes and said anodes of the respective tube there is developed a dflerence of phase of substantially 180, means for connecting the grids of said tubes in parallel with one another, means for connecting said grids to one output terminal 01' the instrument of which an indication is to.

be obtained, means for grounding the other output terminal of said instrument, and means for supplying said instrument with alternating current of a phase and frequency harmonically related to the phase and frequency of the alternat ng current source connected to said cathodes, whereby alternating current appearing between said grids and ground will cause a direct current flow through said indicator connected to said anodes and will indicate phase shift by the direction of said flow, said grids being metallically connected to the respective cathodes through paths of relatively high impedance, whereby sensitivity is maintained at a high level.

2. Electrical indicating device including two electronic tubes, each having at least three elements, including a cathode, an anode and a control grid, means for exciting similar cathodes in the two tubes with a first alternating current so that said cathodes will be substantially 180 out of phase with respect to each other, means for connecting similar control grids in the two tubes in parallel with one another with respect to alternating current to be indicated, said last current being harmonically related, as to phase and frequency, to said first alternating current, means for applying said alternating current to be indicated between said parallel connected pair of control grids and the electrical midpoint of the supply source of said first current, a direct current meter directly and metallically connected between the respective anodes of the tubes so as to be deflected by direct current unbalance of the tubes caused by said alternating current to be indicated, and adjustable resistances interposed in the respective circuits from the anode to the cathode of each tube, whereby direct cur-.

rent flow in said circuits can be matched, sa d parallel connected elements being connected to the respectivecathodes through paths presenting relatively high impedances to said current to be indicated.

JOHN M. VAN BEUREN. ELTON CONRAD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,586,533 Peterson June 1, 1926 1,728,835 Petch Sept 17, 1929 OTHER REFERENCES Alternating Current Bridge Methods, by Hague, 4th ed., 1938, pages 244-246. 

